6" to 10". Strong re enforced frame. Fully sealed solid rail (6 sided) extension eliminates deflection, and delivers the smoothest extension movement available. Removable 1 5/8" stand offs reach around braces. Top plumb vial is permanently positioned at eye level. Frame side opposite the stand offs is a level measuring surface and straight edge. Vials are guaranteed for life never to fog, leak, or require adjustment. Certified accuracy in the top read position: 1/32" over 72".